1. Field of the Invention
The present invention generally relates to a method for reproducing images by digital half-toning and, more particularly, to a reproduction method based upon digital halftoning with error diffusion.
2. State of the Art
In conventional halftone printing, picture elements, or pixels, of a continuous-tone image are converted to dots whose diameters vary in size according to the amplitude of light penetrating a screen. More particularly, the dots are larger where more light penetrates a screen and are smaller where less light is present. Thus, conventional halftone printing processes can be described as being of the analog type since in the processes, dot size is the analog of color intensity for individual picture elements of an original image.
To reproduce multi-color images from continuous-tone originals by conventional halftone printing, a color-filtered screen is prepared for each of four primary colors. Then the colors are printed sequentially while maintaining precise mechanical registration of the screens. Although conventional halftone printing can produce excellent results, the processes involve economies-to-scale and are relatively expensive for short production runs.
Digital halftone printing, in contrast to conventional halftone printing, is better suited for shorter production runs. In digital halftone printing, the initial step is to scan an image with an optical scanner that detects light intensity values for selected colors. Then for each pixel location, the detected data are converted to gray-scale values to provide a basis for image reproduction. Typically, gray-scale values range from 0 to 255.
More particularly, in digital halftone printing, dots are printed in areas corresponding in scale to the original pixel locations. The printed dots normally are of fixed size, but their placement and density within the printed areas are varied in accordance with the detected gray-scale values. Thus, in images that have been produced by digital halftone printing, dot density within each printed area determines the spatial resolution and perceived grayness of the area to an observer.
One shortcoming of conventional digital halftone printing is that printed images often have a grainy or "noisy" appearance. To reduce the grainy appearance of digital halftone images and to improve detail resolution, it has been suggested to use error diffusion techniques in conjunction with digital halftone printing. For example, the text Digital Halftoning, by Robert Ulichney (MIT Press, 1987),
265-268, describes error diffusion techniques with perturbation. Those particular error diffusion techniques are sometimes referred to as spatial dithering.
To apply error diffusion techniques to digital halftone printing, gray-scale values at detected pixel locations are binary coded. Typically, the binary coding threshold is 127, i.e., halfway between the minimum and maximum gray-scale values The binary number "1", for instance, can be assigned to pixel locations where detected gray-scale values are less than 127, and the binary number "0" can be assigned to pixel locations where detected gray-scale values are 127 or more.
As an example of binary coding for error diffusion in digital halftoning, a pixel location with a detected gray-scale value of 100 would be coded as a binary "1". Similarly, a pixel location with a gray-scale value of 145 would be coded as a binary "0". In monochrome printing systems, a pixel location encoded as a binary "1" generally is black, and a pixel encoded as a binary "0" generally is white.
The above-described coding techniques for digital halftoning with error diffusion inherently result in coding errors for each pixel location having a gray-scale value other than 0 or 255. For instance, the magnitude of the coding error resulting from binary encoding a pixel location having a gray-scale value of 100 would be 100 units as measured by gray-scale values. Similarly, the magnitude of the coding error for binary encoding of a pixel location having a gray-scale value of 110 would be 110 units as measured by gray-scale values.
The basic concept underlying digital halftone printing with error diffusion is to distribute, or diffuse, coding errors from the locations at which the errors arise. This concept is further discussed in an article, entitled "An Adaptive Algorithm for Spatial Greyscale", R. W. Floyd and L. Steinberg, Proc. SID, vol. 17/2, pp. 75-77 (1976). Various algorithms have been proposed to accomplish error diffusion.
Although images produced by digital halftone printing with conventional error diffusion techniques may not have a grainy appearance, the printed images often contain discernible patterns. Such patterns are often referred to as "artifacts." Artifacts sometimes appear as straight lines and at other times as wiggly or worm-like shapes. At still other times, artifacts have a random, pebble-like appearance. Regardless of their particular form or shape, however, artifacts detract from the appearance of printed images and therefore limit the commercial usefulness of digital halftoning.